The present invention relates to a temperature-dependent switch, which has a temperature-dependent switching mechanism and a housing which accommodates the switching mechanism and comprises an upper part and a lower part, wherein a first contact area is provided on an inner side of the upper part and a second contact area is provided internally in the lower part, the switching mechanism produces, in temperature-dependent fashion, an electrically conductive connection between the first and second contact areas, the switching mechanism comprises a current transfer element, a bimetallic snap-action disc and a movable contact area, said movable contact area being connected to the current transfer element and interacting with the first contact area, and wherein the bimetallic snap-action disc lifts off the movable contact area from the first contact area depending on the temperature of said bimetallic snap-action disc.
Such a switch is known from DE 10 2011 119 637 A1.
The known switch has a pot-like lower part, which is closed by an upper part engaging over the lower part. A temperature-dependent switching mechanism is arranged in the interior of the switch, said switching mechanism bearing a movable contact part, on which a movable contact area is provided, said contact area interacting with a stationary mating contact, which mating contact is arranged on an inner side of the upper part and forms a first contact area. The first contact area can also be formed directly on an inner side of the upper part.
The switching mechanism comprises, as current transfer element, a spring snap-action disc, which bears the movable contact part and presses said contact part against the stationary mating contact. In the process, the spring snap-action disc is supported with its rim on the inner base of the lower part, which forms the second contact area. In this position, the two contact areas are therefore electrically conductively connected to one another via the movable contact part and the spring snap-action disc.
Contact is made with the known switch from the outside via the electrically conductive cover part, which is electrically conductively connected to the stationary mating contact, and the likewise electrically conductive lower part, with the spring snap-action disc being supported on the inner base thereof.
A bimetallic snap-action disc which lies loosely in the switching mechanism in the low-temperature position of said bimetallic snap-action disc, is arranged above the spring snap-action disc. If the temperature of the bimetallic snap-action disc increases to a value above its response temperature, it presses, with its center, the movable contact part and therefore the movable contact area away from the stationary mating contact, for which purpose it is supported with its rim on an insulating film, which is provided between the lower part and the upper part. The spring snap-action disc in the process snaps over from its one stable geometric configuration to its other stable geometric configuration.
While in the embodiment described to this extent the spring snap-action disc operates against the bimetallic snap-action disc, in the case of the switch known from DE 10 2011 119 637 A1 provision is also made for only a bimetallic snap-action disc to be used, so that the current flows directly through the bimetallic snap-action disc, which also effects the contact pressure between the movable contact part and the stationary mating contact when the switch is closed.
Snap-action discs of the type used here are slightly curved discs with a center which is slightly raised with respect to the rim. The snap-action discs are generally round, circular, oval or similarly rounded, but can also be star-shaped or cross-shaped.
Bimetallic snap-action discs have a high-temperature position, in which they are convex in one view, while they appear concave in the same view when they are in their low-temperature position.
Spring snap-action discs, on the other hand, have two mechanically stable geometric positions or configurations, which appear to be convex or concave depending on the view.
Snap-action discs snap over from their one configuration to the other configuration by virtue of their center moving so to speak through the rim, which strives to perform a radial evasive movement in the process. If the rim is clamped in fixedly, this snapping-over process takes place over internal deformations whilst overcoming internal forces. These internal deformations and the internal forces occurring in the process result in mechanical loading and ageing of the snap-action discs, which limits the life of the switches equipped with said snap-action discs.
In order to avoid or at least significantly reduce the occurrence of the internal deformations and internal forces, the snap-action discs are therefore often prevented from being clamped in mechanically at their rim.
In the switch known from DE 10 2011 119 637 A1, the snap-action disc bearing the movable contact part is, for example, a circular disc, which has an inner contact region, onto which the movable contact part is welded. In order to avoid internal distortions in the snap-action disc, the inner contact region is separated from the snap-action disc by a semicircular gap, which extends over an angle of more than 180°.
A connecting web is formed integrally on the outer rim of the snap-action disc, said connecting web acting, together with the rest of the rim, as second contact region. This connecting web is used for better manipulation of the switching mechanism during fitting thereof and during insertion into the lower part. The connecting web is then welded flat onto the inner base of the lower part in order to ensure a permanent electrical and mechanical connection between the snap-action disc and the second contact area internally in the lower part. The second contact region is thus connected to the second contact area permanently in the region of the connecting web and in the region of the rim when the switch is closed.
This design provides the advantage that the material and production costs for the known temperature-dependent switch are lower than for other switches because no rotary part is required as the lower part and because it is possible to dispense with silver-plating both for the snap-action disc and for the lower part. On the other hand, the complexity involved in fitting is greater than in the case of switches into which the temperature-dependent switching mechanism is merely inserted, as is known from DE 43 45 350 C2.
Owing to the permanent galvanic connection between the snap-action disc and the second contact area, in the switch known from DE 10 2011 119 637 A1 it is ensured that the contact resistance between the snap-action disc and the lower part is very low. In this way, a possible source of faults is eliminated, which may crop up during final continuity testing of a ready-fitted temperature-dependent switch. That is to say that it is quite possible for the contact resistance between the lower part of the housing and the snap-action disc to be so great owing to manufacturing tolerances that the finished temperature-dependent switch needs to be disposed of as a reject.
In a conventional manner, temperature-dependent switches of the type mentioned at the outset are provided with snap-action discs, however, which rest with their rim loosely, i.e. freely movably, on the inner base of the lower part or a shoulder running peripherally internally in the lower part, so that the entire rim forms an outer contact region. Such switches are known, for example, from the above-mentioned DE 43 45 350 A1. During snapping over from one geometric configuration into the other, the snap-action disc is extended until its rim, as it snaps over, lifts off from the base of the lower part or the peripheral rim.
Owing to the fact that the spring snap-action disc is supported on a peripheral shoulder in the switch known from DE 43 45 350 A1, during snap-over it can move with its center “through” the shoulder and its rim resting on the shoulder towards the base which is further below, i.e. snap through the rim while at the same time it extends mechanically radially outwards, which enables snap-over without external mechanical counterforces needing to be overcome.
These mechanical degrees of freedom during snap-over between the two geometric configurations are desirable since they have a positive effect on the life of the switching mechanism and the long-term constancy of the switching temperature.
In order to meet the physical heights and/or the desired function of the individual component parts of such a switch, it is known to arrange a spacer ring between the upper part and the lower part, said spacer ring providing the corresponding accommodating area in the interior of the switch, depending on the height of the temperature-dependent switching mechanism. In accordance with DE 195 27 253 B4, the spacer ring can be in the form of an insulator or a heating resistor, which resistor is electrically connected both to the upper part and to the lower part. This heating resistor is then used for self-holding, as will be described in more detail further below.
Although the switch known from DE 10 2011 119 637 A1 has many advantages in respect of costs and fitting, it does have certain disadvantages as regards the life of the switching mechanism and the long-term constancy of the switching temperature because the snap-action disc is connected mechanically fixedly to the inner base of the lower part via the connecting web at a point on its circumference. This design enables neither radial extension nor unimpeded snap-through of the center of the snap-action disc which is therefore subject to external mechanical forces as it springs over.
The known temperature-dependent switches serve the purpose of protecting an electrical device from an excessively high temperature. For this purpose, the supply current for the device to be protected is conducted through the temperature-dependent switch, wherein the switch is thermally coupled to the device to be protected. At a response temperature which is preset by the snap-over temperature of the bimetallic snap-action disc, the respective switching mechanism then opens the circuit by virtue of the movable contact area being lifted off from the stationary mating contact.
The movable contact area can in this case be formed on a contact part moved by the snap-action disc or directly on the snap-action disc.
In order that the switch does not close again after cooling-down of the device, it is known, for example, from the above-cited DE 195 27 253 B4, to provide a self-holding resistor, preferably a PTC thermistor, in parallel with the temperature-dependent switching mechanism, said self-holding resistor being electrically short-circuited by the temperature-dependent switching mechanism when said switching mechanism is closed. If the switching mechanism now opens, the self-holding resistor takes over part of the current previously flowing and in the process is heated to such an extent that it generates sufficient heat to keep the bimetallic snap-action disc at a temperature which is above the response temperature. This procedure is referred to as self-holding and prevents a temperature-dependent switch from closing again in an uncontrolled manner when the device to be protected cools down again.
While self-heating of the snap-action disc owing to the flowing current is often undesirable in the case of such temperature-dependent switches, switches are also known in which, in addition, a series resistor is provided, which is heated in a defined manner by the flowing operating current of the device to be protected. In the case of an excessively high current flow, this series resistor heats up to such an extent that the critical temperature of the bimetallic snap-action disc is reached. In addition to monitoring of the temperature of the device to be protected, the current flowing can also be monitored in this way, and the switch then has a defined current dependence.
Such switches have stood the test sufficiently well in everyday use. If the switches do not open at the zero crossing of an AC supply voltage or when a DC voltage is applied, arcs form when the movable contact part lifts off from the stationary mating contact and/or when the rim of the current-conducting snap-action disc lifts off from the second contact area and sparks fly.
The arcs formed and sparks produced result in contact erosion and, associated with this over the long term, a change in the geometry of the switching areas of the movable contact part and the stationary mating contact, which, over time, also results in an increase in the volume resistance.
In addition to the contact erosion at the stationary mating contact and the movable contact part, contact erosion also occurs at the contact points where contact resistances form, i.e. between the rim of the snap-action discs, which bear the movable contact part, and the second contact area internally in the housing lower part. Over the course of the switching cycles, this likewise results in an increase in the volume resistance, owing to damage to the rim of the snap-action discs, but this volume resistance should be kept as low as possible in order to keep an undefined influence, which changes over the course of the switching cycles, of the current self-heating on the switching behavior as small as possible.
In particular in the case of high switched currents of, for example, 20 to 50 amperes, the material in the region of the contact resistances is heated considerably, with the result that, owing to the low-resistance design of the switch, the essential heat sources are often not the heat of the component part to be protected but the transfer resistances. As a result, the contact erosion at the contacts and contact areas which are heated in any case increases considerably.
These problems increase even more as the number of switching cycles increases, with the result that the switching behavior of the known switches is impaired over the course of time. Against this background, the life, i.e. the number of permissible switching cycles of the known switches, is limited, wherein the life is also dependent on the contact interruption rating, i.e. the current intensity of the switched currents.
DE 977 187 A therefore proposes that, in the case of a temperature-dependent switching mechanism which has only one bimetallic snap-action disc, said bimetallic snap-action disc is relieved of current flow by virtue of the fact that the movable contact part is connected to the housing of the switch via a sun wheel-shaped metal spider. In this way, the current no longer flows through the bimetallic snap-action disc, but predominantly through the metal spider.
A similar approach is used in AT 256 225 A, in which a copper branching is provided on that surface of the bimetallic snap-action disc which is remote from the stationary mating contact, said copper branching connecting the movable contact part to the housing.
The copper branching and the metal spider do not in any way contribute to the mechanical operation of the switch; in contrast, they need to be moved along by the bimetallic snap-action disc during opening and closing of the switch, i.e. they represent additional mechanical loading for said bimetallic snap-action disc. This results in fatigue and, associated with this, not only an undesired shift in the switching temperature, but also in an impaired opening and closing behavior, which considerably limits the life.
In the case of these switches, the bimetallic snap-action disc does also need to provide the closing pressure of the switching mechanism, but this mechanical loading can be accepted in certain switch types.
Against this background, DE 21 21 802 A proposes arranging a spring snap-action disc in parallel with the bimetallic snap-action disc, said spring snap-action disc producing the closing pressure of the switching mechanism and assisting the snap-over movement of the bimetallic snap-action disc both during opening and during closing. In addition, it also conducts the electrical current. In this way, the bimetallic snap-action disc is relieved of both mechanical and electrical load, with the result that its life is markedly extended.
In the case of this switch there is the problem outlined already at the outset on the basis of the switch known from DE 43 45 350 A1 in respect of the unavoidably forming arcs and sparks which limit the life of the known switches ever more the higher the switched current is.
In the case of the switch known from DE 10 2011 119 637 A1, the contact erosion at the rim of the snap-action disc is reduced by the permanent electrical connection between the snap-action disc and the second contact area, but nevertheless current flows not only via the connecting web but also via the rim of the snap-action disc into the second contact area when the switch is closed, i.e. when the rim of the snap-action disc is supported on the second contact area, with the result that the rim is damaged by contact erosion during opening of the switch, which does not impair the volume resistance, but does impair the mechanical switching behavior and therefore the life.
In order to be able to conduct higher currents via temperature-dependent switches which nevertheless have a long life, a current transfer element in the form of a contact bridge or a contact plate is therefore often used, which is moved by a bimetallic or spring snap-action disc and bears two contact parts, which interact with two stationary mating contacts.
In this way, the operating current of the device to be protected flows from the first mating contact via the first contact part into the contact plate, through said contact plate to the second contact part and from said second contact part into the second mating contact. The snap-action disc is thus free of current and the above-mentioned problems with contact erosion at the rims of the snap-action discs are avoided. However, these switches, as are known from DE 26 44 411 A1 or DE 198 27 113 A1, for example, have a greater physical height than the generic switches and are more complex in design terms.